I tried a couple of programs to see if the values in the existing circuit
were even in the right area. PiEl seemed to be the easiest to play with the
values to get a result that seemed to match what was in the amp originally.
That at least made me feel that there was some hope for the original
component values. I tried another program, SimSmith. It seemed to be
useful for assembling a number of components together and then generating a
smith chart of the result.
As far as the actual tank that I prototyped up, the thinking was 'don't
destroy the original tank playing with the turns and tap points'. Match it
as best as I could, replace it, and then move the tap points and add turns
as needed until the results looked better on the miniVNA. Then, duplicate
it with out all the fiddle points so that it was cleaner looking. I hate
just hacking something in to make it work, but I had to start somewhere.
I would have liked to have gone through the process of engineering up the
system from scratch, math wise, as a training exercise. However, I found
little teaching material that would help me through the process so far
(limited tube experience and in linear amp design). Regrettably, college
stopped short in the area of linear amp design. Any designs were of micro
power at best or in a non RF environment and tubes had pushed off the
training books as, well, technology was marching on.
I may try to build a single band amp with some smaller tubes I have kicking
around here at some point just to put math to practice. I don't want
everything to be brand name in here :-)
--------------------------------------------------
From: "Carl" <km1h@jeremy.mv.com>
Sent: Monday, December 31, 2012 8:26 PM
To: "Chris Miller" <c_miller_1@hotmail.com>; <amps@contesting.com>
Subject: Re: [Amps] looking for assistance with MLA-2500 tank circuit rework
What program did you use to calculate values?
Carl
KM1H
----- Original Message -----
From: "Chris Miller" <c_miller_1@hotmail.com>
To: <amps@contesting.com>
Sent: Friday, December 28, 2012 10:49 PM
Subject: [Amps] looking for assistance with MLA-2500 tank circuit rework
Hi to all,
I have taken on a project that has been bugging me for a bit. I have a
MLA2500 that I picked up that has been retrofitted with a pair of 3cx800
intending to have a transportable amp. From what I have read the tubes
are supposed to be a close match for the original tubes, as far as tube
specs go. However, I have found that the amp on 160 through 40 has to
have the load control set to 1 to get close to load on power. 20 through
10 appeared to work o.k.
I decided to sweep the tank through the antenna, with a 2.7k ohm resistor
in place on the anode to simulate tube load and found that resonances
appeared off on lower bands. Figuring I had a good handle on the
process, I prototyped up a new tank coil to make mistakes on. I then
playing with tap points and turns until I achieved what looked like a
good set of patterns for each band, for the range of tuning.
When I attempted to test my results, on dummy load conditions, all bands
are now showing insufficient capacitance for load on all bands with a
little bit of range at near max capacity on 10 near the upper end of the
band.
Since that seemed to be an epic failure, I must be missing steps in the
process. Is there something I should be watching for or doing so that
what I see as tank resonance actually lands where it should on real world
testing? Am I now at the point where I have to do some trial additions
of capacitance to the load control? Is this amp worth the effort to make
it work properly (realizing it is a bit of a sardine can in there)? Has
anyone else done the retrofit on these amps to the 3cx800 tubes, like
this amp has, and did the amp still resonate as the original tubes did?
Thanks for any help
Chris VE3CEA
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